Yun Yuan Chang

Compressibility and thermal expansion of hydrous ringwoodite with 2.5(3) wt% H2O

Abstract Ringwoodite (γ-Mg2SiO4) is the stable polymorph of olivine in the transition zone between 525-660 km depth, and can incorporate weight percent amounts of H2O as hydroxyl, with charge compensated mainly by Mg vacancies (Mg2+ = 2H+), but also possibly as (Si4+ = 4H+ and Mg2+ + 2H+ = Si4+). We synthesized pure Mg ringwoodite containing 2.5(3) wt% H2O, measured by secondary ion mass spectrometry (SIMS), and determined its compressibility at 300 K by single-crystal and powder X-ray diffraction (XRD), as well as its thermal expansion behavior between 140 and 740 K at room pressure. A third-order Birch-Murnaghan equation of state (BM3 EOS) fits values of the isothermal bulk modulus KT0 = 159(7) GPa and (dKT/dP)P=0 = K′ = 6.7(7) for single-crystal XRD; KT0 = 161(4) GPa and K′ = 5.4(6) for powder XRD, with KT0 = 160(2) GPa and K′ = 6.2(3) for the combined data sets. At room pressure, hydrous ringwoodite breaks down by an irreversible unit-cell expansion above 586 K, which may be related to dehydration and changes in the disorder mechanisms. Single-crystal intensity data were collected at various temperatures up to 736 K, and show that the cell volume V(cell) has a mean thermal expansion coefficient αV0 of 40(4) ×10−6/K (143-736 K), and 29(2) ×10−6/K (143-586 K before irreversible expansion). V(Mg) have α0 values of 41(3) ×10−6/K (143-736 K), and V(Si) has α0 values of 20(3) ×10−6/K (143-586 K) and 132(4) ×10−6K (586-736 K). Based on the experimental data and previous work from 29Si NMR, we propose that during the irreversible expansion, a small amount of H+ cations in Mg sites transfer to Si sites without changing the cubic spinel structure of ringwoodite, and the substituted Si4+ cations move to the normally vacant octahedral site at (½, ½, 0). Including new SIMS data on this and several Mg-ringwoodite samples from previous studies, we summarize volume-hydration data and show that the Mg2+ = 2H+ dominates up to about 2 wt% H2O, where a discontinuity in the volume vs. H2O content trend suggests that other hydration mechanisms become important at very high H2O contents.

Phase relations of Fe-Si alloy in Earth's core

Phase relations of an Fe0.85Si0.15 alloy were investigated up to 240 GPa and 3000 K using in situ X-ray diffraction in a laser-heated diamond anvil cell. An alloy of this composition as starting material is found to result in a stabilized mixture of Si-rich bcc and Si-poor hcp Fe-Si phases up to at least 150 GPa and 3000 K, whereas only hcp-Fe0.85Si0.15 is found to be stable between approximately 170 GPa and 240 GPa at high temperatures. Our extended results indicate that Fe0.85Si0.15 alloy is likely to have the hcp structure in the inner core, instead of the previously proposed mixture of hcp and bcc phases. Due to the volumetric dominance of the hcp phase in the hcp + bcc coexistence region close to the outer-core conditions, the dense closest-packed Fe-Si liquid is more relevant to understanding the properties of the outer core.

Elasticity of cubic boron nitride under ambient conditions

As a superhard material with properties similar to diamond including chemical inertness, cubic boron nitride (cBN) is an excellent candidate as a pressure calibration standard for high-temperature high-pressure research using its pressure-volume-temperature (P-V-T) equation of state. However, the elastic properties of cBN at ambient conditions reported in the literature vary by up to 8%, which can likely be attributed in part to variability in the cBN composition and defect structure and measurement uncertainties. We have measured the single-crystal elastic moduli of high-purity cBN with high precision by Brillouin scattering measurements, making an effort to minimize experimental uncertainties. We obtain values of C11 = 798.4 ± 1.7 GPa, C44 = 469.0 ± 1.0 GPa, and C12 = 172.4 ± 1.1 GPa, from which the isotropic aggregate bulk modulus Ks = 381.1 ± 1.3 GPa and shear modulus G = 398.8 ± 1.2 GPa (the Hill average) were calculated. Our results improve the precision and reduce the uncertainties in the elastic m...

Comparative compressibility of hydrous wadsleyite and ringwoodite: Effect of H2O and implications for detecting water in the transition zone

Review of recent mineral physics literature shows consistent trends for the influence of Fe and H2O on the bulk modulus (K0) of wadsleyite and ringwoodite, the major phases of Earths mantle transition zone (410–660 km). However, there is little consensus on the first pressure derivative, K0′ = (dK/dP)P=0, which ranges from about 4 to >5 across experimental studies and compositions. Here we demonstrate the importance of K0′ in evaluating the bulk sound velocity of the transition zone in terms of water content and provide new constraints on the effect of H2O on K0′ for wadsleyite and ringwoodite by conducting a comparative compressibility study. In the experiment, multiple crystals of hydrous Fo90 wadsleyite containing 2.0 and 0.25 wt % H2O were loaded into the same diamond anvil cell, along with hydrous ringwoodite containing 1.4 wt % H2O. By measuring their pressure-volume evolution simultaneously up to 32 GPa, we constrain the difference in K0′ independent of the pressure scale, finding that H2O has no effect on K0′, whereas the effect of H2O on K0 is significant. The fitted K0′ values of hydrous wadsleyite (0.25 and 2.0 wt % H2O) and hydrous ringwoodite (1.4 wt % H2O) examined in this study were found to be identical within uncertainty, with K0′ ~3.7(2). New secondary-ion mass spectrometry measurements of the H2O content of these and previously investigated wadsleyite samples shows the bulk modulus of wadsleyite is reduced by 7.0(5) GPa/wt % H2O, independent of Fe content for upper mantle compositions. Because K0′ is unaffected by H2O, the reduction of bulk sound velocity in very hydrous regions of transition zone is expected to be on the order of 1.6%, which is potentially detectible in high-resolution, regional seismology studies.

Development of a self-organized neuro-fuzzy model for system identification

It has been known that it is difficult to establish a fuzzy logic model with effective fuzzy rules and the associated membership functions. Neural network with its learning capability has been incorporated to make the fuzzy model more adaptive and effective. A self-organized neuro-fuzzy model by integrating the Mamdani fuzzy model and the back-propagation neural network is developed in this paper for system identification. The five-layer network adaptively adjusts the membership functions and dynamically optimizes the fuzzy rules. A benchmark test is applied to validate the model accuracy in nonlinear system identification. Experimental verifications on the dynamics of a composite smart structure and on an acoustics system also demonstrate that the neuro-fuzzy model is superior to the neural network and to an adaptive filter in system identification. The model can be established systematically and is shown to be effective in engineering applications.

Elastic and mechanical softening in boron-doped diamond

Alternative approaches to evaluating the hardness and elastic properties of materials exhibiting physical properties comparable to pure diamond have recently become necessary. The classic linear relationship between shear modulus (G) and Vickers hardness (HV), along with more recent non-linear formulations based on Pugh’s modulus extending into the superhard region (HV > 40 GPa) have guided synthesis and identification of novel superabrasives. These schemes rely on accurately quantifying HV of diamond-like materials approaching or potentially exceeding the hardness of the diamond indenter, leading to debate about methodology and the very definition of hardness. Elasticity measurements on such materials are equally challenging. Here we used a high-precision, GHz-ultrasonic interferometer in conjunction with a newly developed optical contact micrometer and 3D optical microscopy of indentations to evaluate elasticity-hardness relations in the ultrahard range (HV > 80 GPa) by examining single-crystal boron-doped diamond (BDD) with boron contents ranging from 50–3000 ppm. We observe a drastic elastic-mechanical softening in highly doped BDD relative to the trends observed for superhard materials, providing insight into elasticity-hardness relations for ultrahard materials.